The present invention relates to a spreading head, particularly for spreading thermoplastic material on an intermediate component.
Known spreading heads are currently used which are usually arranged transversely to the direction of movement of the intermediate component.
Such heads are usually composed of a laminar-flow assembly having a beak-shaped cross-section, an upper clamp block, and an underlying lower clamp block.
As an alternative, above the lower clamp block there can be a single device that acts as laminar-flow assembly and as upper clamp block.
The thermoplastic material (for example adhesives, including those known as “hot melt” or “reactive hot melt” adhesives) is first melted by means of a suitable continuous melting unit (tank) or a drum unloader and is then injected into the spreading head by means of gear pumps.
The transverse distribution of the adhesive is performed by means of a closed duct affected by uniformly spaced valves (modules), which allow the adhesive to reach a second laminar-flow region through channels and optionally a third region.
Such channels convey the adhesive directly to the laminar-flow assembly, which is directly connected to the intermediate component.
As mentioned, the passage of the fluid adhesive is allowed by suitable valves, usually known as modules, which are mostly actuated by electric valves of the pneumatic or magnetic type.
The duct is closed downstream of each valve by means of a device of the needle type, which closes onto an appropriately provided cup.
The ducts are opened by lifting the needle from the closed position.
The main drawback of this known head is the fact that sometimes the opening movement of the valve is not achieved; this can depend on various situations, including the fact that the injected hot-melt adhesives can be unstable and, if they remain at high temperature, can trigger charring (cracking) phenomena that block the valve.
Furthermore, if reactive hot-melts are used, the adhesive can polymerize in the head itself, consequently blocking the valve.
The opening movement of the valve might not be achieved also due to normal mechanical problems (hindered sliding movements) or due to wear or failure of sealing gaskets (O-rings) of the pneumatic circuit.
Clearly, the uncontrolled failure of a valve to open entails a severe damage for the application of the adhesive and entails, in the case of spreading heads of the known type, a reduction in the overall amount of adhesive applied to the substrate.
The effect in heads of the so-called “step” type (which comprise multiple regions for releasing the material in film form) is much more severe: such heads are choked transversely and each individual valve supplies a step without thereby any possibility to compensate for the adhesive that does not arrive from the unintentionally “closed” valve; the occurrence of this condition entails the uncontrolled production of rejects, since a band as wide as the step of adhesive will be missing on the strip being processed.
In addition to this, in many cases visual monitoring of the application is not possible owing to the small amount of adhesive that is applied: by way of example, the following application is cited:    thickness of applied adhesive: 1.5 microns    characteristics of adhesive: transparent    width of substrate: 3,600 mm    speed of application: 400 m/min
In a correlated way, one can deduce that in the above cited application case the reject produced inadvertently might reach approximately 86,400 square meters/hour.
Moreover, one should consider that the product on which the hot melt is spread is normally coupled immediately by calendering to another film (immediately in order to utilize the ability of the hot melt to act as a bonding agent when it is at high temperature).
This entails the following further remarks: first of all, it is very difficult to perform visual inspection after spreading, since the region is located 300–400 mm from the calendering point (a distance that is covered in 0.0525 s in the above cited operating conditions).
Furthermore, it is extremely difficult to perform inspection, since the region to be examined is normally protected by barriers according to EC standards; finally, the defect (lack of adhesive on a band) is difficult to detect on the coupled material, which is wound immediately in a roll.
Currently, it is known, as a control measure that is usually adopted, to perform a sample check of the production, analyzing a transverse strip taken from the composite during production.
It is noted that this operation can occur easily only on the end portions of the rolls obtained once they have been removed from the machine; otherwise, the removal of samples would entail an inevitable machine downtime (complete cutting of the strip being processed and subsequent splicing).
Besides, identifying a band of defective product says nothing regarding the starting point and end point of the defect: one should bear in mind that since one is speaking of coupled products it is quite difficult to identify the head and the tail of the reject.
Clearly, there is the risk of not identifying a reject despite the sample check.